What Happens to Solar Power When the Sun Doesn't Shine?

The easiest retort for critics of solar and wind power is a smirking, “But what happens when the wind doesn’t blow, and the sun doesn’t shine?” This anti-renewable angle is fairly effective, because the honest answer to that question is: No one knows quite yet.

One solution is to link renewable projects to power plants fueled by a clean energy source, usually natural gas. Another is to store energy generated by solar or wind projects. Some solar projects include vats of molten salt, which, once warmed by solar energy, retains heat. Storing renewable energy in batteries has proved a popular idea, although building and disposing of large batteries leads to environmental problems like lead pollution in developing countries.

But one reason it’s not clear how to adapt the electricity grid to the intermittency of renewables is we're still missing information about how technologies like solar panels behave. After spending the past year studying the topic, the Department of Energy just released one of the first sets of data that describes how clouds affect solar power installations.

The data collected provides at least one new insight, according to the department. It's common sense that when clouds pass over one solar panel or a small rooftop solar system, solar panels’ energy output plunges sharply. When the sun returns, energy output shoots up. But the researchers discovered that the effect of clouds on larger groups of linked solar panels is a calmer tide of energy—a “smoothing of the fluctuation,” as the department puts it. The energy output still varies, but not in so dramatic a fashion that grid and utility operators would have to scramble to react to changes at a moment’s notice.

To collect the data, researchers with the National Renewable Energy Laboratory set up 17 measurement stations on the Hawaiian island of Oahu. For a year, the stations measured the amount of solar energy that reached them, in concert and at one-second intervals. The stations showed cloud movement, and the resulting data can be used to model the behavior of solar installations producing up to 30 megawatts of electricity. (The largest solar installations in the United States have grown beyond that capacity, but a 30 MW project can still power thousands of homes.) With more information about cloud behavior, the researchers say, utilities can begin building software and other tools to help the grid adapt to the patterns of power that clouds can create.

The DOE did not pick Hawaii at random to host the study. The state has to ship in the fossil fuels it uses to power its tourism and military industries, so solar-generated electricity could make financial sense there sooner than in other states. And unlike some renewable-crazy places, like Seattle or Portland, Hawaii has no shortage of sunlight.